1. Field of the Invention
The present invention relates to rotary cylinder apparatus of a video tape recorder (VTR) by which a video signal is recorded on or reproduced from a magnetic tape, and particularly to a driving motor of rotary cylinder apparatus.
2. Description of the Prior Art
As is well known, the rotary magnetic head VTR is designed to record or reproduce signals onto or from a magnetic tape by high-speed rotation of the magnetic heads and, in order to reproduce or record clear and stable video images, the rotary magnetic heads must be very smoothly rotated at quite constant speed.
Recently, there have been proposed rotary cylinder apparatus of the direct-drive type in which the rotary shaft of the motor is directly connected to the magnetic heads and the motor used is a brushless motor with long life and less vibration.
This type of rotary cylinder apparatus, is disclosed in, for example, U.S. Pat. No. 4,117,519 and comprises a rotary drum having mounted thereon two rotary heads and a primary rotary transformer directly connected to the heads, with a stationary drum opposing the rotary drum and on the top of which there is mounted a secondary rotary transformer through a first wiring board.
The brushless motor for driving the rotary drum is placed at the lower portion of the stationary drum under the first wiring board, and the motor mainly comprises a stator yoke of a magnetic material fixedly attached to the first wiring board, a driving coil mounted on the stator, and a permanent magnet rotor opposed to the driving coil through a small gap in the axial direction. A detector section for operation of the brushless motor comprises a position detector having a position detecting rotor disposed at the lower portion of the permanent magnet rotor, a position detecting stator mounted on the stationary drum to oppose the position detecting rotor, a second wiring board for wiring lead wires from the position detecting stator, and a revolution-rate detector having a revolution-rate detecting coil mounted in a housing which is secured to the lower end of the stationary drum, a permanent magnet for magnetic excitation, and stator teeth supported to oppose a rotor teeth mounted on the shaft.
The conventional apparatus of such construction can be small-sized and made light in weight as compared with the belt-drive type because the shaft of the motor is directly connected to that of the rotary drum.
However, the driving coil and position detecting stator, as main parts of the motor, require first and second wiring board, and it is also necessary to separately produce the stationary drum and the housing and to fixedly connect together by bolts or the like because the revolution-rate detecting coil is mounted within the housing at the lower end of the stationary drum. Therefore, it is difficult for the conventional apparatus to be small-sized.
Moreover, the revolution-rate detector is a generator or tachometer comprising rotor teeth formed in a gear-like shape on the outer periphery of the exciting permanent magnet which is fixed to the rotating shaft, stator teeth opposing to the rotor teeth and a revolution-rate detecting coil supported by a magnetic member, whereby the change of permeance upon rotation of the rotor teeth is detected by the revolution-rate detecting coil.
This generator requires a great number of teeth because of the need for high precision, and thus the rotor teeth and stator teeth become too large to be incorporated in the interior of the motor. Therefore, the requirements for small size and high precision, which are incompatible with each other, can not be fully satisfied.
Morever, the assembly of such apparatus takes much time because parts to be assembled must be disposed at several different places and the mounting of parts on chassis or the like also takes much time and labor.